Mirror substrates used in projection optics systems of extreme ultraviolet lithography (EUVL) scanners must meet stringent thermal expansion requirements in order to maintain their original surface shape (known as “figure”) when subjected to temperature changes caused by exposure to high power illumination during normal operation of the scanner. A temperature independent figure is necessary to avoid thermally-induced distortions in the wavefront characteristics of EUV projection optics. For this reason, the preferred material for EUVL mirror substrates is Ultra Low Expansion glass (ULE® Glass), manufactured by Corning Incorporated. Glass sold by Corning Inc. under the glass code 7973 is specifically tuned for EUVL applications. Corning EUVL glasses are characterized with high degrees of precision and accuracy to properly identify mirror substrates that are narrowly targeted to specific applications.
A defining feature of ULE® Glass is the existence of a temperature close to room temperature at which the coefficient of thermal expansion (CTE) is exactly equal to zero. This temperature is known as the crossover temperature, the zero-crossover temperature, or temperature of zero crossover of the glass and is denoted Tzc. Another important feature of ULE® glass for EUVL is that the slope of the temperature-dependent CTE curve (CTE slope) is extremely small within a temperature range close to room temperature that includes Tzc. The CTE slope of ULE® glass is in the vicinity of 1×10−9/K2 (or, equivalently, 1 ppb/K2). EUVL mirror substrates having Tzc near the temperatures expected when the mirror substrate is exposed to an EUV optical source experience minimal thermal expansion during operation of the EUVL scanner and a small CTE slope ensures that the minimal thermal expansion is preserved if fluctuations in EUVL processing conditions cause variations in the thermal environment of the mirror substrate.
As EUVL technology advances, it is expected that higher energy optical sources will be employed to increase system productivity. The semiconductor industry is also expected to improve the efficiency of chip manufacturing processes by adopting larger wafer sizes (e.g. 450 mm), which increases duty cycle and thus the range of mirror temperature variations. The push to reduce feature size and increase device density will require scanners with higher numerical aperture (NA), which translates into an increase in the size of mirror substrates used in EUVL scanners. As the size of mirror substrates increases, the requirements for uniformity of Tzc and CTE slope will become increasingly stringent and more challenging to achieve. As EUV optical sources become more powerful and operate at new wavelengths, it will also be necessary to develop mirror substrates that maintain desirable Tzc and CTE slope characteristics over a wider range of thermal environments.
In order to meet the needs of the EUVL industry, it is desirable to develop glasses and manufacturing processes that enable control over Tzc and CTE slope. Control over Tzc and CTE slope can provide for systematic variations in Tzc and CTE slope in glass samples extracted from different parts of a boule (or other large glass monolith) so that a single boule can be used to provide all of the mirror substrates needed to accommodate the range of thermal environments experienced by EUVL mirrors at different positions within a typical EUVL scanner. Mirror substrate manufacturing efficiency can be improved if multiple substrates can be extracted from each manufactured glass boule or monolith and the Tzc or CTE slope of each mirror substrate can be adjusted to meet the specific requirements of different mirror components in an EUVL scanner.
The prior art teaches that control of the fictive temperature, Tf, of ULE® Glass can be used to tune Tzc within a narrow range while simultaneously reducing CTE slope. A shortcoming of the prior art, however, is that control of the single parameter Tf simultaneously varies both Tzc and CTE slope. In the methods of the prior art, Tzc and CTE slope are coupled and cannot be independently tuned. As a result, the glass manufacturer has been forced to finely tune the glass forming process to yield a glass composition such that, once Tf is controlled to a certain value, both CTE slope and Tzc will be within the range required by the target application. Mirror substrates produced by prior art methods are therefore usable only within a narrow range of operating conditions within an EUVL scanner. To enlarge the range of operating conditions using prior art processing methods, it is necessary to prepare multiple glass boules or monoliths that differ in glass composition. Relying on compositional variations to meet the needs of EUVL technology is inconvenient, costly, and time consuming. There is a need for new processing methods that permit independent control of Tzc and CTE slope for a given glass composition over a wide range of values.